Proposals have already been made to perform such detection by means of thermocouples. However, hot zones in energy transmission cables are generally not more than a few meters long, which means that a very large number of thermocouples need to be distributed along the entire length of the cable.
Document U.S. Pat. No. 3,510,762 proposes disposing an auxiliary cable along an electrical energy cable with the auxiliary cable including a duct full of a liquid which is vaporizable in the vicinity of the maximum admissible temperature for the energy cable and whose vapor has a dielectric constant which is markedly different from that of the liquid, together with a high-frequency cable for detecting points at which the liquid is vaporized by reflectometry using electric pluses. However, such apparatus is too expensive for ordinary use, and it is only sensitive to the boiling temperature.
Also, document U.S. Paat. No. 3,187,080 proposes apparatus for detecting the temperature in different lengths of an electric cable which includes a tube filled with a liquid that is vaporizable in the vicinity of the maximum admissible temperature for the cable and in which each junction in the cable has a chamber connected to said tube, the chamber being partially filled with the same vaporizable liquid and being provided with a pressure-operated switch. The effect of partial vaporization of the liquid in the tube causes the liquid level to rise in the chamber and a float actuates the switch, thereby short-circuiting a pair of telephone wires incorporated in the cable and thus indicating which zone of the cable is being heated abnormally. Such apparatus cannot detect the zone where excessive heating is taking place with much precision, particularly since its indications are disturbed by temperature variations in the remainder of the cable outside the zone which is too hot. Further, the apparatus of the above document, like the previous apparatus, indicates that the vaporization temperature has been reached by detecting a variation in volume by virtue of the liquid vaporizing at constant pressure.
The object of the present invention is to provide apparatus for continuously measuring the temperature of a hot point on an electrical energy transmission cable with greater accuracy, and also providing indications which are more reliable than those of prior apparatuses.